1. Field of the Invention
The present invention relates to an apparatus for use in forming multilayered paper in a paper making machine.
2. Description of the Related Art
FIG. 3 shows an example of a prior art device for use in forming multilayered paper. The device shown in FIG. 3 is located at the end of a wet part of a typical paper making machine. Paper stock ejected from a head box 20 is dehydrated on a table unit 22 located within the loop of a short wire 21. A paper layer is formed on the short wire 21 when the stock is further dehydrated by pressing the stock with felt 19 pressed against a forming cylinder 24. A paper layer formed by each upstream unit U and the paper layer(s) formed by the downstream unit(s) D are bonded to each other on the forming cylinder 24 of the downstream unit by being pressed with a coacting roll 25. The white water spouting from the forming cylinder 24 when the short wire 21 is separated from the forming cylinder 24 is drawn out by a suction slider 26. On the other hand, the paper layers bonded on the forming cylinder 24 are shifted to the felt 19 by a suction pick-up 29 so as to be fed to the next (downstream) unit.
FIG. 4 shows another example of a prior art device for use in forming multilayered paper. This device is a typical part of paper making machine which combine layers of paper by using short wires and a Fourdrinier wire. In FIG. 4, paper stock ejected from a head box 30 is formed into a paper layer by being dehydrated on a table unit 33 disposed within the loop of a Fourdrinier wire 32. In addition, paper stock ejected from a head box 31 lands on the paper layer already formed and enters into a gap formed by a short wire 34 engaged with a lead-in box 40 of the Fourdrinier wire. In the portion where the short wire coacts with the Fourdrinier wire, the stock from the head box 31 is concurrently formed into a paper layer and combined with the already formed paper layer by being dehydrated in an upward direction due to tension in the short wire in pressing engagement with the lead-in box 40 and a vacuum produced at a vacuum dehydrator 39.
After that, the combined paper layers are shifted solely to the Fourdrinier wire 32 by the vacuum created by a transfer box 41 whereupon the combined paper layers are transferred to a downstream short wire 45. In order to increase the density of the combined paper layers, a vacuum box 42 is provided within the loop of the Fourdrinier wire. Paper comprising the paper layers thus combined is picked up by a suction pick-up roll 44 downstream of a suction couch roll 43 and is subjected to a subsequent process.
The multilayered paper is not sufficiently formed by the device shown in FIG. 3 because the initial paper layer was formed by a table unit 22 (such as a forming board, foil, table roll and a wet suction box of the type known for use in a paper making machine employing the Fourdrinier wire and cylinder mold) and the paper layers are dehydrated by forming cylinders 24.
In addition, in this prior art device, the wet mat can be crushed due to the difficulty in taking the wet paper into the forming cylinder at a high speed. This problem becomes more severe as the wet paper consistency decrease with increases in the amount of stock per layer on the short wire. Accordingly, the amount of stock per layer in the short wire cannot be increased without risking the problem of the wet mat being crushed at the forming cylinder. Further, the device shown in FIG. 3 is intended to facilitate white water treatment. However, there are problems created by the spattering of mist. Further, the white water in the cylinder mold is difficult to treat because the felt travels over the forming cylinder 24.
Replacement of the felt 19 in this prior art device is difficult because the loop formed by the felt is complicated due to the fact that the felt 19 is Wrapped around portions of a plurality of short wires 21. Accordingly, sometimes felt having a seam (similar to that of drawing canvas) is used, and a mark on the paper can occur when the seam arrives at the coacting roll 25.
Further, in the device for forming multilayered paper shown in FIG. 3, the bonding strength of the wet mat decreases as the dehydrating capacity of the short wire table is increased which raises the wet paper density.
In the prior art device for use in forming multilayered paper shown in FIG. 4, a high level of vacuum is required to suction the white water upwardly against gravity and to discharge the white water from the autoslice provided in the vacuum dehydrator. Because the white water always fills the vacuum dehydrator 39, an even greater level of vacuum was required for discharging the white water and dehydrating the stock in the two wires through the white water. In addition, because the short wires 34, 45 are pressed forcefully on the forming shoe (not shown) of the vacuum dehydrators 39 by the high level of vacuum, a large amount of power was required to drive the short wires.
With respect to the ability to operate a low speed and to make high weight paper, the prior art device of FIG. 4 is better than that of the prior art device of FIG. 3 because the radius of curvature at the lead-in box in FIG. 4 is larger than that at the forming cylinder in FIG. 3. However, there are still some problems in leading the wet paper layer into the short wire units at a very low speed in the prior art device of FIG. 4. Therefore, it is necessary to use a relatively large amount of stock per layer.
Further, in the short wire units in the prior art device of FIG. 4, treating the white water from the short wire 34 is difficult and measures are required for preventing the mist generated from spattering, because the short wire units are provided on the Fourdrinier wire.